Seamless steel tubes and pipes for use in oil well

ABSTRACT

Disclosed are seamless steel tubes for oil well use, comprising C: 0.14-0.35%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Cr: 0.05-1.5%, Mo: 0.05-2.0%, Ti: 0-0.05%, V: 0-0.1%, and Al: not less than 0.010%, wherein the concentration product by Al and N content, corrected by Ti and V, is within the range of 0.00001 to 0.00050, and the residuals are Fe and impurities including P: 0.025% or less, and S: 0.010% or less. Ti, V, Nb, or B is preferably contained to enhance the quench hardenability as well as the resistance to sulfide stress corrosion cracking, and further Ca, Mg and/or REM is preferably contained to improve the shape of non-metallic inclusions, enhancing the resistance to sulfide stress corrosion cracking. Thus, said tubes by the invention can be produced by efficient means realizing energy savings, and widely used as ones having excellent stability in mechanical strength.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to seamless steel tubes and pipes (hereinafter, simply referred to as tube(s)) for use in oil well, more particularly, to tubes having less variation of tensile strength, i.e., excellent stability in mechanical strength.

2. Description of the Related Art

Seamless steel tubes having higher reliability compared to welded tubes, are frequently used in hostile oil well environment and/or high temperature environment, so that the tubes having higher mechanical strength in association with stable strength, higher toughness and higher resistance to sour gas environment degradation are required.

For instance, in Japanese Patent Application Publication No. 2001-73086, seamless steel tubes containing V, Nb, Ti, Cr and Mo that are arranged to satisfy the relationship expressed by the preset equation to thereby suppress the formation of M₂₃C₆ type carbide, which are in general formed in quenching and tempering treatment, are disclosed as having high toughness as well as high corrosion resistance.

In order to meet these requirements, it is essential to stabilize the mechanical strength, especially in steel tubes that require the resistance to sour gas environment degradation, the narrower scatter band of mechanical strength is mostly concerned. Also, even ordinary oil well tubes are likely to be deformed partially due to the outer pressure in oil well, which is attributed to the variation of mechanical strength (scatter of mechanical strength), so that the stability of mechanical strength in steel tubes becomes an indispensable factor.

Conventionally, in order to stabilize the mechanical strength, it is perceived that the uniform tempered martensite microstructure is preferable for steel tubes, and, thus, C, Mn, Cr, Mo and the like are required to be controlled within narrow allowable range to precisely adapt the quench hardenability. However, this much measure could not realize the stable mechanical strength. What is more, an in-line heat treatment is taking place of a conventional off-line heat treatment from the viewpoint of an enhancement of productivity, the savings of energy in use and cost reduction. In this in-line heat treatment, there is an issue that the quench hardenability fluctuates according to the variation of grain size of prior austenite, thus making it difficult to obtain the stable mechanical strength.

As afore-mentioned, there is still left a problem to be solved in order to produce seamless steel tubes for use in oil well having stable mechanical strength, high toughness and resistance to sour gas corrosion environment, wherein the variation of quench hardenability attributable to the variation of prior austenite grain size is suppressed.

SUMMARY OF THE INVENTION

The present invention is started in view of the above circumstances, and the task thereof is to provide seamless steel tubes for use in oil well having excellent stability in mechanical strength, which can be produced by efficient means leading up to realization of energy saving.

The present inventors, to achieve the above task, looked into the problems thus far and made various investigations on the production of seamless steel tubes having excellent stability in mechanical strength to thereby obtain findings shown in (a)-(d) in the followings, and, thus, the present invention is completed.

(a) In order to reduce the variation of mechanical strength of steel due to the variation of quench hardenability of steel tube, it is important to control the precipitation amount of AlN within steel. In this regard, it is necessary to control the concentration product (hereinafter, may be alternatively referred to as “Al×{N−14×(Ti/144+V/153)}”), as an index of AlN precipitation amount, to be expressed by Al content (mass %) and the effective N content (mass %) corrected with Ti and V each as a nitride former, within the range of 0.00001 to 0.00050.

(b) The reason why the concentration product, Al×{N−14×(Ti/144+V/153)}, is controlled within the range indicated in the above (a) is shown as below. Namely, when the above concentration product factor exceeds 0.00050 and gets higher, the AlN precipitation amount increases to suppress the coarsening of grain size to thereby reduce the quench hardenability of steel tube. On the other hand, when the above concentration product is less than 0.00001, the AlN precipitation amount decreases to coarsen the grain size to thereby enhance the quench hardenability of steel tube. That such a possible fluctuation of quench hardenability of steel tube to be created as above shall be suppressed to stabilize the mechanical strength shall necessitate to control the above concentration product within the range indicated in the above (a).

(c) In order to secure the mechanical strength of steel tube upon enhancing the quench hardenability and to prevent the toughness from decreasing, it is effective to control each content of C, Si, Mn and Cr, while it is effective to control Mo content in order to enhance the quench hardenability and the resistance to sulfide stress corrosion cracking.

(d) When Ti is contained, N is immobilized as nitride to thereby leave B in the dissolved state, so that the quench hardenability can be enhanced. Also, when V is contained, fine carbide precipitates during tempering treatment to thereby enhance the mechanical strength. Nb contributes to enhance the resistance to sulfide stress corrosion cracking by forming carbonitride, and so does B by enhancing the quench hardenability which leading up to increasing martensite. Further, by containing one or more of Ca, Mg and REM, the shape of non-metallic inclusions can be improved to enhance the resistance to sulfide stress corrosion cracking.

The present invention is completed based on the above findings and the gist thereof pertains to seamless steel tubes for use in oil well shown in the following (1)-(4).

(1) Seamless steel tubes for use in oil well, comprising, by mass %, C: 0.14-0.35%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Cr: 0.05-1.5%, Mo: 0.05-2.0%, Ti: 0-0.05%, V: 0-0.1%, and Al: not less than 0.010%, wherein each content of Al, N, Ti and V satisfies the relationship expressed by the equation (i) below, and wherein the residuals are Fe and impurities including P: not more than 0.025% and S: not more than 0.010%, 0.00001≦Al×{N−14×(Ti/144+V/153)}≦0.00050  (i),

where each symbol of elements in equation (i) denotes the content (mass %).

(2) Seamless steel tubes for use in oil well, comprising, by mass %, C: 0.14-0.35%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Cr: 0.05-1.5%, Mo: 0.05-2.0%, Ti: 0-0.05%, V: 0-0.1%, and Al: not less than 0.010%, further comprising one or two of Nb: 0.005-0.040% and B: 0.0003-0.005%, wherein each content of Al, N, Ti and V satisfies the relationship expressed by the equation (i) above, and wherein the residuals are Fe and impurities including P: not more than 0.025% and S: not more than 0.010%.

(3) Seamless steel tubes for use in oil well, comprising, by mass %, C: 0.14-0.35%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Cr: 0.05-1.5%, Mo: 0.05-2.0%, Th: 0-0.05%, V: 0-0.1%, and Al: not less than 0.010%, further comprising one or more of Ca: 0.0003-0.005%, Mg: 0.0003-0.005% and REM: 0.0003-0.005%, wherein each content of Al, N, Ti and V satisfies the relationship expressed by the above equation (i), and wherein the residuals are Fe and impurities including P: not more than 0.025% and S: not more than 0.010%.

(4) Seamless steel tubes for use in oil well, comprising, by mass %, C: 0.14-0.35%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Cr: 0.05-1.5%, Mo: 0.05-2.0%, Ti: 0-0.05%, V: 0-0.1%, and Al: not less than 0.010%, further comprising one or two of Nb: 0.005-0.040% and B: 0.0003-0.005%, and also one or more of Ca: 0.0003-0.005%, Mg: 0.0003-0.005% and REM: 0.0003-0.005%, wherein each content of Al, N, Ti and V satisfies the relationship expressed by the above equation (i), and wherein the residuals are Fe and impurities including P: not more than 0.025% and S: not more than 0.010%.

In the present invention, “excellent stability in mechanical strength” is meant to that there is less variation of strength like tensile strength of steel, which will be recited at the later stage.

Besides, mass % may be designated by simply %, hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the effect of Al and the effective N on the variation of yield strength of steel tubes that were subjected to quenching and tempering treatment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention pertains to seamless steel tubes for use in oil well, comprising C: 0.14-0.35%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Cr: 0.05-1.5%, Mo: 0.05-2.0%, Ti: 0-0.05%, V: 0-0.1%, and Al: not less than 0.010%, wherein each content of Al, N, Ti and V satisfies the relationship expressed by foregoing equation (i), and wherein the residuals are Fe and impurities including P: not more than 0.025% and S: not more than 0.010%. Herein, the reason for such limitation as above in the invention and a preferable range is recited further.

C:

Carbon (C) is contained for the purpose of ensuring the mechanical strength of seamless steel tubes for use in oil well. When its content is less than 0.14%, the quench hardenabiliy becomes deficient. Thus, the temperature of tempering treatment can not be raised, so that it becomes difficult to ensure the required property of steel. Meanwhile, when its content exceeds 0.35%, quench cracking likely occurs and the toughness is also deteriorated. By reason of the above, the proper range of C content is set to 0.14-0.35%. Incidentally, the preferable range of C content is 0.16-0.28%, and more preferably 0.20-0.28%.

Si:

Silicon (Si) has not only a function of a deoxidizer but also a function of enhancing the quench hardenability of steel to heighten the mechanical strength.

In order to utilize the above function, it is necessary for Si of not less than 0.05% to be contained. However, when its content exceeds 1.0% and gets higher, the resistance to sulfide stress corrosion cracking deteriorates. Thus, the proper range of Si content is set to 0.05-1.0%. Further, the preferable range of Si content is 0.1-0.5%.

Mn:

Manganese (Mn) has not only a function of a deoxidizer but also a function of enhancing the quench hardenability of steel to heighten the mechanical strength. In order to utilize the above function, it is necessary for Mn of not less than 0.05% to be contained. However, when the content exceeds 2.0% and gets higher, the compositional segregation aggravates to thereby decrease the toughness. Thus, the proper range of Mn content is set to 0.05-2.0%.

P:

Phosphor (P) is an impurity in steel and tends to segregate along the grain boundaries to thereby reduce the toughness. Accordingly, the proper range of P content is set to not more than 0.025%. And a preferable P content is not more than 0.020%.

S:

Sulfur (S) is also an impurity in steel, and tends to unite with Mn or Ca to form non-metallic inclusions. When S content exceeds 0.010%, the deterioration of the toughness as well as the resistance to sulfide stress corrosion cracking in steel worsens, which is attributable to non-metallic inclusions. Thus, the proper range of S content is set to not more than 0.010%. And a preferable S content is not more than 0.005%.

Cr:

Chromium (Cr) is an effective element in enhancing the quench hardenability of steel, and needs to be contained by not less than 0.05% in order to achieve the above effect. However, when its content exceeds 1.5% and gets higher, the toughness as well as the resistance to sulfide stress corrosion cracking in steel decreases. Thus, the proper range of Cr content is set to 0.05-1.5%. And a preferable Cr content is 0.2-1.2%.

Mo:

Molybdenum (Mo) is an effective element not only in enhancing the quench hardenability to thereby ensure the high mechanical strength but also in increasing the resistance to sulfide stress corrosion cracking. In order to achieve the above effect, Mo should be contained by not less than 0.05%. However, when its content exceeds 2.0% and gets higher, the toughness as well as the resistance to sulfide stress corrosion cracking in steel decreases. By reason of the above, the proper range of Mo content is set to 0.05-2.0%. And a preferable Mo content is 0.1-0.8%.

Al:

Aluminum (Al) has a function of a deoxidizer and is an effective element in enhancing the toughness as well as the workability of steel. And when Al content is less than 0.010%, the dissolved state C increases, resulting in the marked increase of mechanical strength. Thus, the proper range of Al content is set to not less than 0.010%. And a preferable upper limit of Al content is 0.080%.

Reason for limitation of each of Al, N, Ti and V content by the relationship expressed by the equation (i):

The reason why the concentration product, Al×{N−14×(Ti/144+V/153)}, to be given by Al content (mass %) and the effective N content, which is afore-mentioned, is controlled to fall within the range expressed by the equation (i) below is described in the followings. 0.00001≦Al×{N−14×(Ti/144+V/153)}≦0.00050  (i)

When the above concentration product, Al×{N−14×(Ti/144+V/153)}, which is an index to indicate the extent how much AlN precipitation is present in steel, exceeds 0.00050 and gets higher, the amount of AlN precipitation in steel increases. Consequently, the coarsening of grain size is suppressed and the quench hardenability decreases. On the other hand, the concentration product is less than 0.00001, the amount of AlN precipitation in steel decreases and grain size is coarsened, resulting in the increase of quench hardenability. By reason of the above, in order to suppress the fluctuation of quench hardenability in association with the decrease or excessive increase of quench hardenability to thereby stabilize the mechanical strength, a proper range of the concentration product is set to 0.00001-0.00050 as expressed by the above equation (i). And a preferable range of the concentration product is 0.00003-0.0003.

FIG. 1 is the diagram showing the effect of Al and the effective N on the variation of yield strength of steel tubes that were subjected to quenching and tempering treatment.

In this diagram, the valuation whether the variation of yield strength of steel tubes is large or small is made in accordance with the equation (ii) as described hereinbelow. Namely, the symbol ∘ in the diagram designates that the variation of mechanical strength is small, while the symbol ● designates that the variation of mechanical strength is large.

As seen from said diagram, the proper range of the concentration product to be given by Al content (mass %) and the effective N content is the region where the above equation (@) is satisfied, and also Al content is not less than 0.010%.

In the followings, a first and a second group of chemical elements and compositions which are to be contained, when necessary, are set forth.

A first group consists of Ti, V, Nb and B, wherein one or more of these elements is contained, when necessary.

Ti:

Titanium (Ti) is an element having the function that it immobilizes N in steel as nitride to thereby leave B in steel as the dissolved state during quenching treatment, thus contributing to enhance the quench hardenability. This element may not be contained, but the above function can be utilized by containing it.

However, when Ti content exceeds 0.05% and gets higher, it remains in steel as coarse nitride to thereby reduce the resistance to sulfide stress corrosion cracking.

By reason of the above, the range of the content of Ti when it is contained is set to 0-0.05%. And a preferable range of the content is 0.005-0.025%.

B:

Boron (B) is an element having the function that it contributes to enhance the quench hardenability to thereby increase the amount of martensite, thus enhancing the resistance to sulfide stress corrosion cracking. Even if its content should be the level of an impurity, its function can be exerted. It does not matter whether it may be contained or not, but it is preferable that its content is not less than 0.0003% in order to exhibit the more distinct effect. When B content exceeds 0.005% and gets higher, however, the steel toughness decreases. Thus, a content range when B is contained is set to 0.0003-0.005%. And a preferable content range is 0.0003-0.003%.

V:

Vanadium (V) is an element having the function that it precipitates as carbide during tempering treatment to thereby heighten the mechanical strength of steel. This element may not be contained, but the above function can be utilized by containing it. On the other hand, when its content exceeds 0.3% and gets higher, the steel toughness decreases. From this reason, a content range when V is contained is set to 0-0.3%.

Nb:

Niobium (Nb) is an element having the function that it forms carbonitride at an elevated temperature range to thereby prevent the grain size from coarsening, thus enhancing the steel toughness as well as the resistance to sulfide stress corrosion cracking. This element may not be contained, but the above function can be utilized by containing it by not less than 0.005%. On the other hand, when its content exceeds 0.040% and gets higher, carbonitride gets excessively coarsened to thereby deteriorate the resistance to sulfide stress corrosion cracking. Thus, a content range when Nb is contained is set to 0.005-0.040%. And a preferable content range is 0.010-0.030%.

A second group consists of Ca, Mg and REM.

It does not matter whether these elements may be contained or not, but when contained, either of these has the function that it reacts with S in steel to form sulfide to thereby improve the shape of the non-metallic inclusion, thus enhancing the resistance to sulfide stress corrosion cracking. When the above function needs to can be utilized, one or more elements being selected out of Ca, Mg and REM (Rare Earth Metal element such as Ce, La and Y) is contained by an amount of not less than 0.0005% in each. On the other hand, when either content exceeds 0.005% and gets higher, non-metallic inclusions in steel increase to thereby deteriorate the resistance to sulfide stress corrosion cracking. By reason of the above, the content range when either element is contained is set to 0.0005-0.005%.

EXAMPLES

In order to confirm the effect of seamless steel tubes for use in oil well according to the present invention, following tests were carried out and the results were evaluated.

The billet of 225 mm in diameter was made for each of 22 grades of test steel whose chemical compositions are shown in Tables 1 and 2. Then, the billet was heated at 1250° C., and subjected to Mannesmann Mandrel Tube Making Process to obtain a seamless steel tube of 244.5 mm in outside diameter with 13.8 mm in thickness. Thereafter, the seamless steel tube was subjected to quenching and tempering treatment and the tensile test specimen was sampled therefrom. TABLE 1 Inventive or Chemical Compositions (mass %, Residuals of Fe and Impurities) Test No. Steel No. Comparative C Si Mn P S Cr Mo Al N 1 1 Inventive Steel 0.24 0.25 0.92 0.012 0.002 0.51 0.48 0.010 0.0050 2 2 Inventive Steel 0.27 0.09 0.81 0.023 0.006 0.60 1.93 0.010 0.0100 3 3 Inventive Steel 0.24 0.35 1.21 0.018 0.009 0.75 0.52 0.010 0.0500 4 4 Inventive Steel 0.15 0.75 1.30 0.018 0.006 1.23 0.31 0.030 0.0020 5 5 Inventive Steel 0.25 0.44 0.63 0.018 0.004 0.06 0.52 0.030 0.0050 6 6 Inventive Steel 0.21 0.35 1.12 0.009 0.006 0.60 0.56 0.030 0.0160 7 7 Inventive Steel 0.24 0.35 0.89 0.018 0.006 0.98 0.52 0.050 0.0012 8 8 Inventive Steel 0.22 0.93 1.95 0.014 0.006 0.60 0.05 0.050 0.0050 9 9 Inventive Steel 0.24 0.36 1.30 0.018 0.006 1.10 0.58 0.050 0.0100 10 10 Inventive Steel 0.31 0.35 0.12 0.018 0.006 0.60 0.72 0.070 0.0010 11 11 Inventive Steel 0.23 0.55 0.71 0.024 0.006 1.47 0.52 0.070 0.0020 12 12 Inventive Steel 0.24 0.31 1.30 0.018 0.006 0.60 0.52 0.070 0.0070 13 13 Comparative Steel 0.19 0.35 1.30 0.018 0.006 0.60 0.52  0.009* 0.0060 14 14 Comparative Steel 0.22 0.35 1.30 0.023 0.006 0.60 0.74  0.009* 0.0110 15 15 Comparative Steel 0.27 0.35 1.30 0.018 0.004 0.53 0.52  0.009* 0.0560 16 16 Comparative Steel 0.21 0.35 1.30 0.018 0.006 0.60 0.59  0.009* 0.0670 17 17 Comparative Steel 0.24 0.35 1.30 0.016 0.006 0.60 0.97 0.010 0.0600 18 18 Comparative Steel 0.30 0.35 1.30 0.019 0.006 0.60 0.75 0.030 0.0200 19 19 Comparative Steel 0.26 0.35 1.30 0.018 0.003 1.23 0.23 0.050 0.0120 20 20 Comparative Steel 0.24 0.35 1.30 0.017 0.006 0.89 0.52 0.070 0.0090 21 21 Comparative Steel 0.28 0.35 1.30 0.018 0.005 0.75 0.89 0.038 0.0043 22 22 Comparative Steel 0.24 0.35 1.30 0.018 0.006 0.60 0.52 0.026 0.0046 Note: The symbol * denotes the deviation from the specified range by the present invention.

TABLE 2 Inventive or Chemical Compositions (mass %, Residuals of Fe and Impurities) Value of Variation Test No. Steel No. Comparative Ti V Nb B Ca Mg REM Equation (i) of YS 1 1 Inventive Steel — — — — — — — 0.000050 ∘ 2 2 Inventive Steel — — — — — — — 0.000100 ∘ 3 3 Inventive Steel — — — — — — — 0.000500 ∘ 4 4 Inventive Steel — — — — — — — 0.000060 ∘ 5 5 Inventive Steel — — — — — — — 0.000150 ∘ 6 6 Inventive Steel 0.002 — — 0.0030 — 0.0030 — 0.000474 ∘ 7 7 Inventive Steel — 0.01 0.010 — — — — 0.000014 ∘ 8 8 Inventive Steel — 0.05 — — 0.0003 — — 0.000021 ∘ 9 9 Inventive Steel — — — — 0.0031 — 0.001 0.000500 ∘ 10 10 Inventive Steel 0.008 — 0.036 0.0003 — 0.0003 — 0.000016 ∘ 11 11 Inventive Steel — — — — — — — 0.000140 ∘ 12 12 Inventive Steel — — — — — — — 0.000490 ∘ 13 13 Comparative Steel — — — — — — — 0.000054 x 14 14 Comparative Steel — — — — — — — 0.000099 x 15 15 Comparative Steel 0.015 — — — — — 0.004 0.000491 x 16 16 Comparative Steel — — 0.012 — — — —  0.000603* x 17 17 Comparative Steel — 0.12 — 0.0008 0.0024 — —  0.000510* x 18 18 Comparative Steel — — — — — 0.0006 —  0.000600* x 19 19 Comparative Steel — — — 0.0020 — — 0.004  0.000600* x 20 20 Comparative Steel — — — — — — —  0.000630* x 21 21 Comparative Steel 0.010 0.04 — — — — — −0.000013* x 22 22 Comparative Steel 0.020 0.10 — — — — — −0.000169* x Note: The symbol * denotes the deviation from the specified range by the present invention. The definition of symbols ∘ and x is described in the text of this specification.

Herein, quenching was performed in such a way that, after holding 5 min at 950° C. for uniform heat distribution, water quenching was applied, and tempering was performed by holding the steel tube thus quenched at 650° C. for 30 min. The above heat treatment condition is one example, and the quenching and tempering treatment to be applied for seamless steel tubes according to the present invention is not limited to the above.

And, with regard to tensile test, the tensile test specimen with circular arc strip shape cross-section at its parallel portion, being specified in 5CT in API Standard, is prepared from the longitudinal direction of the sampled tube coupon for tensile test purpose, and then, the tensile test was conducted to measure yield strength YS (MPa).

Further, the variation of yield strength YS based on the measured results as above was assessed to evaluate the stability in mechanical strength, which was shown in Table 2 also.

In addition, in Table 2, the variation of yield strength YS was checked by following method to evaluate by two-step technique. Namely, 10 tensile tests (N frequency=10) as above were conducted for each test steel designated by Steel No., and the test result was checked with the below equation (ii). When the equation was satisfied, it was judged that the variation of yield strength was small and satisfactory (∘), while, when the equation was not satisfied, it was judged that the variation of YS was large and defective (x). Each YS≦Average YS−3×Standard Deviation of Each YS  (ii)

Herein, “Average YS” is defined to designate the average YS in tensile tests across all test steels (22 grades) to be used for testing, “Each YS” is defined to designate the individual average YS in tensile tests for the test steel (one grade) of interest, and “Standard Deviation of Each YS” is defined to designate the standard deviation for the test steel of interest (one grade).

Test Nos. 1-12 are Inventive Examples which employed Steel Nos. 1-12 according the present invention, while Test Nos. 13-22 are Comparative Examples which employed Steel Nos. 13-22 that are comparative steels.

In Test Nos. 13-15 that employed Steel Nos. 13-15 of less Al content, and in Test No. 16 that employed Steel No. 16 that had less Al content and high value of the concentration product factor or Al×{N−14×(Ti/144+V/153)}, each variation of yield strength YS was large, and the stability in mechanical strength was poor and unsatisfactory. Also, in Test Nos. 17-22 that employed Steel Nos. 17-22 where Al content was within the specified range by the present invention, but the concentration product or Al×{N−14×(Ti/144+V/153)} was high, the variation of yield strength YS in either test was large, and the stability in mechanical strength was poor and unsatisfactory.

By contrast, in Test Nos. 1-12 that employed Steel Nos. 1-12 that conform to all conditions specified by the present invention, the variation of yield strength YS was small, and the stability in mechanical strength was excellent. Especially, in Test Nos. 6, 7, 8 and 10 that employed Steel Nos. 6, 7, 8 and 10 that contained Ti, V, Nb or B, much higher mechanical strength as well as the resistance to sulfide stress corrosion cracking were obtained, due to enhancement of quench hardenability, while in Test Nos. 6, 8, 9 and 10 that employed steels with one or more of Ca, Mg and REM, much higher resistance to sulfide stress corrosion cracking were obtained.

INDUSRIAL APPLICABILITY

The present invention provides seamless steel tubes for oil well use having excellent stability in mechanical strength, which can be produced by efficient means leading up to realization of energy saving. In particular, even when a process utilizing in-line heat treatment is applied, the fluctuation of quench hardenability due to the variation of prior austenite grain size is suppressed to thereby enable the stable and high mechanical strength to be obtained. Therefore, seamless steel tubes for oil well use according to the present invention is highly appreciated as ones having the stability in mechanical strength which can be produced under the circumstances where energy savings along with productivity is mostly concerned, and can be widely used from the viewpoint of both streamlining of production process and the expansion of application. 

1. Seamless steel tubes for use in oil well, comprising, by mass %, C: 0.14-0.35%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Cr: 0.05-1.5%, Mo: 0.05-2.0%, Ti: 0-0.05%, V: 0-0.1%, and Al: not less than 0.010%, wherein each content of Al, N, Ti and V satisfies the relationship expressed by the equation (i) below, and wherein the residuals are Fe and impurities including P: not more than 0.025% and S: not more than 0.010%, 0.00001≦Al×{N−14×(Ti/144+V/153)}≦0.00050  (i), where each symbol of elements in equation (i) denotes the content (mass %).
 2. Seamless steel tubes for use in oil well, comprising, by mass %, C: 0.14-0.35%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Cr: 0.05-1.5%, Mo: 0.05-2.0%, Ti: 0-0.05%, V: 0-0.1%, and Al: not less than 0.010%, further comprising one or two of Nb: 0.005-0.040% and B: 0.0003-0.005%, wherein each content of Al, N, Ti and V satisfies the relationship expressed by the equation (i) below, and wherein the residuals are Fe and impurities including P: not more than 0.025% and S: not more than 0.010%, 0.00001≦Al×{N−14×(Ti/144+V/153)}≦0.00050  (i), where each symbol of elements in equation (i) denotes the content (mass %).
 3. Seamless steel tubes for use in oil well, comprising, by mass %, C: 0.14-0.35%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Cr: 0.05-1.5%, Mo: 0.05-2.0%, Ti: 0-0.05%, V: 0-0.1%, and Al: not less than 0.010%, further comprising one or more of Ca: 0.0003-0.005%, Mg: 0.0003-0.005% and REM: 0.0003-0.005%, wherein each content of Al, N, Ti and V satisfies the relationship expressed by the equation (i) below, and wherein the residuals are Fe and impurities including P: not more than 0.025% and S: not more than 0.010%, 0.00001≦Al×{N−14×(Ti/144+V/153)}≦0.00050  (i), where each symbol of elements in equation (@) denotes the content (mass %).
 4. Seamless steel tubes for use in oil well, comprising, by mass %, C: 0.14-0.35%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Cr: 0.05-1.5%, Mo: 0.05-2.0%, Ti: 0-0.05%, V: 0-0.1%, and Al: not less than 0.010%, further comprising one or two of Nb: 0.005-0.040% and B: 0.0003-0.005%, and also one or more of Ca: 0.0003-0.005%, Mg: 0.0003-0.005% and REM: 0.0003-0.005%, wherein each content of Al, N, Ti and V satisfies the relationship expressed by the equation (i) below, and wherein the residuals are Fe and impurities including P: not more than 0.025% and S: not more than 0.010%, 0.00001≦Al×{N−14×(Ti/144+V/153)}≦0.00050  (i), where each symbol of elements in equation (i) denotes the content (mass %). 